NATURAL PHILOSOPHY. 1;39 



course, not translucent, but wlien they are intendorl to be manu- 

 factured in the form of kiti'hen and domestic utensils the varnish 

 is omitted. They are, moreover, covered with a variety of de- 

 signs, produced by corrodinir the surface of the glass and platin- 

 izing the engraved portions, which, therefore, are ren(h.'red alone 

 transparent. Very beautiful and elaborate designs can be pro- 

 duced in this manner. One of the distinguishing features char- 

 acterizing the light transmitted by glasses platinized in the manner 

 describetl, is its peculiar softness and tone. M. Leroux was the 

 first to notice this particular attribute of the light, and stated that it 

 might be turned to good service in shielding the vision when en- 

 gaged in regarding any intense source of heat, such as the sun, 

 smeltino^ or g^as furnaces. AVhen the natural sijilit is weak or 

 temporarily deranged, these platinized glasses might be advan- 

 tageously substituted for the tinted or colored ones usually em- 

 l^loyed, which are supposed to possess powers of neutralization 

 that in reality rarely belong to them. They have already been 

 replaced by the former in some astronomical instruments, to mod- 

 ify the intensity of the solar rays. All that is necessary is to place 

 one of the glasses before the ol)ject-glass of the telescope, by 

 which means a large proportion of the rays are retiected, and only 

 a sufficient number pass through to enable the observer to study 

 the aspect of the luminous body, without fatigue or annoyance to 

 the eye. This property of sui)duing and softening rays of ardent 

 light is not confined to platinized glass. The same effect is pro- 

 duced by the application of different metallic substances. If a 

 pale-blue glass be simplj' covered with a piece of gold leaf, the 

 light transmitted is instantl}' endowed with a peculiar soft tone. 

 A slight characteristic tint is also imparted to the light, which de- 

 pends upon the nature of the metal employed. Thus, if pure gold 

 be used, the tint is of a light-greenish hue, whih^ the ordinary or 

 jeweller's gold, which always contains a certain proportion of 

 silver, gives a bluish shade, varying in depth of color with the 

 amount of alloy in the gold. The effect of thin sheets of metallic 

 substances upon light has been known for a long period, and M. 

 Foucault has proposed to silver the object-glasses of telescopes 

 employed solely for taking observations of the sun. He himself 

 made the experiment upon the lens of a large telescope in the 

 French lloval Observatorv, and found that the image lost none of 

 its clearness or sharpness, and the plan was greatly superior to the 

 ordinary one of interposing a colored medium before the eye-glass 

 of the instrument. — Mechanics'' Magazine. 



WHAT ARE BRAZILIAN PEBBLES ? 



They are nothing but pure crystallized silica without a flaw, and 

 are not necessarily all Brazilian ; but as they have been found 

 plentifully and ver}^ perfect in that country, a trade has sprung up 

 for the supply of the London and Paris lens manufacturers. Spec- 

 tacle lenses were formerly all made of common glass, but they 

 have been found to be much less pure, and at the same time to be 



